Développement d'outils de reconstruction et sensibilité du démonstrateur SuperNEMO

SuperNEMO is an experiment looking for the neutrinoless double beta decay in an effort to unveil the Majorana nature of the neutrino. The first module, called the demonstrator, is under construction and commissioning in the Laboratoire Souterrain de Modane. Its unique design combines tracking and calorimetry techniques. The demonstrator can study 7 kg of ⁸²Se, shaped in thin source foils. These source foils are surrounded by a wire chamber, thus allowing a 3-dimensional reconstruction of the charged particles tracks. The individual particles energies are then measured by a segmented calorimeter, composed of plastic scintillators coupled with photomultipliers. A magnetic field can be applied to the tracking volume in order to identify the charge of the particles. SuperNEMO is thus able to perform a full reconstruction of the events kinematics and to identify the nature of the particles involved: electrons, positrons, α particles or γ particles. In practice, the particle and event reconstruction relies on a variety of algorithms, implemented in the dedicated SuperNEMO simulation and reconstruction software. The γ reconstruction is particularly challenging since γ particles do not leave tracks in the wire chamber and are only detected by the calorimeter, sometimes multiple times. Several γ reconstruction approaches were explored during this thesis. This work lead to the creation of a new algorithm optimizing the γ reconstruction efficiency and improving the γ energy reconstruction. Other programs allowing the particle identification and performing the topological measurements relevant to an event were also developed. The value of the magnetic field was optimized for the 0νββ decay search, based on Monte-Carlo simulations. The magnetic shieldings performances and their impact on the shape of the magnetic field were estimated with measurements performed on small scale magnetic coils. The SuperNEMO demonstrator is able to measure its own background contamination thanks to dedicated analysis channels. At the end of the first 2.5 years data taking phase, the main backgrounds target activities should be measured accurately. The ⁸²Se 2νββ half-life should be known with a 0.3 % total uncertainty. Unlike other double beta decay experiments relying solely on the two electrons energy sum, SuperNEMO has access to the full events kinematics and thus to more topological information. A multivariate analysis based on Boosted Decision Trees was shown to guarantee at least a 10 % increase of the sensitivity of the 0νββ decay search. After 2.5 years, and if no excess of 0νββ events is observed, the SuperNEMO demonstrator should be able to set a limit on the 0νββ half-life of T > 5.85 10²⁴ y, translating into a limit on the effective Majorana neutrino mass mββ 10²⁶ y or mββ 5.85 10²⁴ ans, équivalant à une limite supérieure sur la masse effective du neutrino mββ 10²⁶ ans et mββ < 40 − 110 meV